Field of the Invention
The invention relates to a specimen holder for a Charged Particle Microscope, and in particular to specimen holders with improved versatility and a larger number of degrees of freedom of motion.
Description of the Related Art
The invention relates to a Charged Particle Microscope comprising such a specimen holder, and to a method of using such a Charged Particle Microscope.
Charged particle microscopy is a well-known and increasingly important technique for imaging microscopic objects, particularly in the form of electron microscopy. Historically, the basic genus of electron microscope has undergone evolution into a number of well-known apparatus species, such as the Transmission Electron Microscope (TEM), Scanning Electron Microscope (SEM), and Scanning Transmission Electron Microscope (STEM), and also into various sub-species, such as so-called “dual-beam” tools (e.g. a FIB-SEM), which additionally employ a “machining” Focused Ion Beam (FIB), allowing supportive activities such as ion-beam milling or Ion-Beam-Induced Deposition (IBID), for example. More specifically:                In a SEM, irradiation of a specimen by a scanning electron beam precipitates emanation of “auxiliary” radiation from the specimen, in the form of secondary electrons, backscattered electrons, X-rays and photoluminescence (infrared, visible and/or ultraviolet photons), for example; one or more components of this flux of emanating radiation is/are then detected and used for image accumulation purposes.        In a TEM, the electron beam used to irradiate the specimen is chosen to be of a high-enough energy to penetrate the specimen (which, to this end, will generally be thinner than in the case of a SEM specimen); the flux of transmitted electrons emanating from the specimen can then be used to create an image. When such a TEM is operated in scanning mode (thus becoming a STEM), the image in question will be accumulated during a scanning motion of the irradiating electron beam.More information on some of the topics elucidated here can, for example, be gleaned from the following Wikipedia links:        en.wikipedia.org/wiki/Electron_microscope        en.wikipedia.org/wiki/Scanning_electron_microscope        en.wikipedia.org/wiki/Transmission_electron_microscopy        en.wikipedia.org/wiki/Scanning_transmission_electron_microscopyAs an alternative to the use of electrons as irradiating beam, charged particle microscopy can also be performed using other species of charged particle. In this respect, the phrase “charged particle” should be broadly interpreted as encompassing electrons, positive ions (e.g. Ga or He ions), negative ions, protons and positrons, for instance. As regards ion-based microscopy, some further information can, for example, be gleaned from sources such as the following:        en.wikipedia.org/wiki/Scanning_Helium_lon_Microscope        W. H. Escovitz, T. R. Fox and R. Levi-Setti, Scanning Transmission Ion Microscope with a Field Ion Source, Proc. Nat. Acad. Sci. USA 72(5), pp. 1826-1828 (1975).It should be noted that, in addition to imaging, a charged particle microscope may also have other functionalities, such as performing spectroscopy, examining diffractograms, performing (localized) surface modification (e.g. milling, etching, deposition), etc.        
In all cases, a Charged Particle Microscope (CPM) will comprise at least the following components:                A radiation source, such as a Schottky electron source or ion gun.        An illuminator, which serves to manipulate a “raw” radiation beam from the source and perform upon it certain operations such as focusing, aberration mitigation, cropping (with an aperture), filtering, etc. It will generally comprise one or more (charged-particle) lenses, and may comprise other types of (particle-)optical component also. If desired, the illuminator can be provided with a deflector system that can be invoked to cause its output beam to perform a scanning motion across the specimen being investigated.        A specimen holder, on which a specimen under investigation can be held and positioned (e.g. tilted, rotated). If desired, this holder can be moved so as to effect scanning motion of the beam w.r.t. the specimen. In general, such a specimen holder will be connected to a positioning system such as a mechanical stage.        A detector (for detecting radiation emanating from an irradiated specimen), which may be unitary or compound/distributed in nature, and which can take many different forms, depending on the radiation being detected. Examples include photomultipliers (including solid state photomultipliers, SSPMs), photodiodes, CMOS detectors, CCD detectors, photovoltaic cells, etc., which may, for example, be used in conjunction with a scintillator film, for instance.In the case of a transmission-type microscope (such as a (S)TEM, for example), the CPM will also comprise:        An imaging system, which essentially takes charged particles that are transmitted through a specimen (plane) and directs (focuses) them onto analysis apparatus, such as a detection/imaging device, spectroscopic apparatus (such as an EELS module), etc. As with the illuminator referred to above, the imaging system may also perform other functions, such as aberration mitigation, cropping, filtering, etc., and it will generally comprise one or more charged-particle lenses and/or other types of particle-optical components.In what follows, the invention may—by way of example—sometimes be set forth in the specific context of electron microscopy. However, such simplification is intended solely for clarity/illustrative purposes, and should not be interpreted as limiting.        
A specimen under investigation in a CPM is generally located in a very cramped space, in very close proximity to the terminal optical elements of the CPM's illuminator. In the case of a dual-beam CPM, this situation is exacerbated by the fact that there are two optical columns—e.g. one for electrons and one for ions—which converge (from different directions) at the specimen, thereby causing even greater cramping. In addition, the CPM may employ a gas injection system and/or micromanipulator(s), which will further crowd the vicinity of the specimen. In the case of a transmission-type CPM, available space is even more confined, since the first optical elements of the imaging system are located just below the specimen. Such cramped conditions have led to the development of rod-like specimen holders, on which a specimen mounting zone is located at/near one extremity (second end) of a relatively thin elongated member, which is fine enough to be introduced laterally (so called “side entry”) into the cramped specimen space described above. The other extremity (first end) of this elongated member is connected to a support structure (e.g. a simple structure such as a knob or handle, or a composite structure comprising, for example, a dewar for containing a cryogenic coolant), and this support structure is generally intended to remain outside a retaining wall of the CPM's vacuum enclosure while said connected elongated member protrudes through an aperture in said wall. In many such cases, a portion of the elongated member inside the CPM will seat into a cradle that is connected to an actuator system (e.g. the so-called CompuStage in TEMs supplied by FEI Company), allowing the elongated member (and a specimen mounted thereon) to be positioned/moved in multiple degrees of freedom relative to the/an optical axis of the CPM. To aid clarity, a Cartesian coordinate system will be adhered to in this discussion, in which:                The (longitudinal axis of the) elongated member extends along the X direction;        The/an optical axis of interest in a particular investigation extends (temporarily) along the Z direction.In such a system, said actuated cradle will, for example, be positionable in X, Y, Z. It will also often be positionable in Rx (rotation about X, also called alpha tilt or roll), e.g. so as to allow a tilt series (sinogram) to be acquired during (TEM) tomography. In multi-beam (e.g. dual-beam) CPMs in which a plurality of particle-optical axes (co-planar in a plane O) converge on a specimen space, the elongated member of the specimen holder is conventionally arranged to as to extend perpendicular to O; in this way, Rx (alpha tilt) positioning can be used to “present” an exposed surface of a clamped specimen to a given particle-optical axis, at will.        
In addition to imaging, an important aspect of working with a CPM is specimen preparation. This is particularly (though not exclusively) the case in transmission-type CPMs, in which the specimen will generally be extremely thin (e.g. of the order of 1-100 nm), consequently relatively brittle/delicate, and therefore (very) difficult to work with. Once such a specimen is (precariously) mounted on the (specimen mounting zone of the) holder (e.g. using adhesive, or a mechanical clamping mechanism such as a clip, flange, screw, etc.), it is highly desirable not to have to demount it until strictly necessary. Nevertheless, after mounting, many operations (alterations, finishing) may have to be performed on the specimen, such as ion milling, ion-beam-induced deposition (IBID), electron-beam-induced deposition (EBID), etc., for purposes of thinning, surface modification, etc. Many such functionalities can be made available in situ in a CPM, but their applicability/usefulness is limited in many situations by sub-optimal manipulability of the specimen holder. In this context, the current inventors have worked extensively to identify shortcomings in conventional holder designs, and to address these effectively so as to produce better performance. The results of this endeavor are the subject of the current invention.